MORTARS IN HISTORIC B U I L D I ~AGREVIEW S. OF THE CONS~I<VATION, TECIINILAL AND SCIENT~FIC LITERATURE
co~lsideredthe best, as such mixtures occupy space in the most efficient way (Torraca 1995).
The contact surface of rnoulds with the specimens, suction and entrance of air influences maiilly limebased mortars (Charola & Henriques 1999). Therefore, properties of lirne mortar made in a steel mould are different from those of Inortar cast between masonry (Lawrence & Samarasinghe, 2000). They depend mainly on the mould surface, the size of the specimen and how long the specimen was left in the mould. Mortar specimens for investigation of properties should be prepared in a realistic way. Lawrence & Samarasinghe (2000) suggested producing and curing mortar specimens between two masonry units.
Type of aggregate, its grading, type of lime (age of lime putty) and liinelaggregate proportions control the amount of water needed to provide good workability. The optimal waterlbinder proportions differ depending on construction and application techniques. Good compaction of liine mortar is vital for its performance. Good workable lime mortar possesses a greater degree of plasticity; it is often described as similar to a modelling clay.
The mixing and production methods of lime mortars can also have a very strong influence on their performance. Maturing of some lime putties reduces their particle size and improves their water retentivity (Hansen et al. 2000). The sand carrying capacity of lime mortar should improve with reduction of particle size (Gibbons 1995 TAN 1). Hand and different mechanical mixing can produced mortars of different quality. From practice it is known that the mortar plasticity can be improved by the method of mixing. Traditional techniques of mixing 'by hand' involved beating, chopping and ramming on a wooden board until the mix was sticky and workable (Gibbons 1995 TAN 1). Ready-mixed mortar should be re-mixed before use. In relation to the performance of mortars assessed by standard testing, Henriques & Charola (1996) assessed the effect of mixing and preparation on compressive strength of pozzolanic lime mortars. The two standard procedures prescribed by British Standard (BS 4551) and European specifications (EN 196-1) resulted in pozzolanic inortar specimens with different mechanical properties. The pure liine mortar specimens, however, did not show any significant differences in their mechanical properties for these two
different preparation and mixing standards. The mechanical properties of both non-hydraulic and pozzolanic mortars were also affected by different curi~lgconditions prescribed by British Standard (BS 455 1 ) and French specifications (CSTB). Wetter conditions (BS 4551) favoured hydration but slowed down carbonatio~l(Henriques & Charola (1996).
The curinglagei~igconditions for non-hydraulic lime mortars proinote a combination of drying out and carbonation at such a rate that minimises shrinkage. The ideal environment to achieve a maximum rate of carbonation, has a temperature around 20째C and relative humidity between 50-70% (Van Balen & Van Gemert 1994). The strength development of limebased mortars due to carbonation is inherently a very long-term process, depending on the curinglageing conditions. Curing described by British Standard (BS 455 1: Part 1: 1998) for mortar specimens is not appropriate for nonhydraulic lime mortars. It assumes the presence of a hydraulic binder, which requires a damp environment or immersion to allow hydration of the hydraulic components. Moist curing in a container can possibly be used, however, the container should not be airtight. According to literature (Parrot 1991-1992) and considering the nature of the hardening process of lime mortar, such humid conditions can retard carbonation and do not represent ambient conditions encountered in practice. The common 28 days' curing is not sufficie~itfor nonhydraulic lime mortars (Cliarola and Henriques 1999). A length of curing to provide a comparable strength testing can be difficult to predict. It depends on the curing conditions, moulding and size of the sample. Accelerated curing where the specimens were exposed to a higher CO, concentration was described (Knofel and Schubert 1993). This accelerated curing was developed to make testing of lime mortars available at 28 days. The effects of the specimen size and the length of curing on the strength testing of lime mortars have been observed in Inany research papers (e.g. Baronio ef al. 2000).
5.4.3. Fresh nzortars Mortars, while fresh, are tested in order to ensure a certain standard quality. The tests are described in detail in national standards (e.g. British Standard BS 4551: Part 1 Methods of testing mortars, screeds and plasters) EN 1015 Methods of tests for Mortar for Masonry 1999. Of these tests, the workability test is the most beneficial for practice. It combines needs for appropriate consistency of the material regarding its